1. Field of the Invention
The invention relates to a process for reactivating catalysts which are used in a fluid-bed process for the ammoxidation of olefins. The reactivation is carried out by addition of a reactivator.
Important ammoxidation processes carried out on a large scale produce acrylonitrile or methacrylonitrile from propylene or i-butene by reaction with ammonia and oxygen. The ammoxidation catalysts hitherto available and used on a large scale can retain their high initial activities and particularly their high initial selectivities only for a few weeks, and in many cases even for only a few days, and then show a marked decrease which especially concerns the desired selectivity to (meth)acrylonitrile, whereas the total conversion of the olefin to increasingly appearing undesired by-products, such as CO, CO.sub.2, acetonitrile or hydrogen cyanide, remains in general unchanged.
2. Description of the Related Art
To limit the yield losses of the desired nitriles, various methods for reactivating the ammoxidation catalysts have been developed. Thus, in DE-OS (German Published Specification) 2,435,031, a reactivator is used which is formed from 85% by weight of MoO.sub.3 and 15% by weight of SiO.sub.2 and which can raise the yield of 63% of acrylonitrile by addition of 6.4% by weight of reactivator, relative to the deactivated catalyst, to 70% acrylonitrile yield.
High additions of molybdenum oxide are also used in the process of DE-OS (German Published Specification) 3,311,521; in the process mentioned, these amount to about 2.2% by weight, relative to the deactivated ammoxidation catalyst, and the acrylonitrile yield can rise, for example, from 65.5% to 72.5%.
A further reactivation method is described in DE-OS (German Published Specification) 2,717,579, the deactivated ammoxidation catalyst being treated with an impregnation solution which contains molybdenum and bismuth in a molar ratio of 0.5-20:1 and optionally phosphoric acid. The treated catalyst is calcined and then used again. In the optimum case, the selectivity for acrylonitrile is here raised from 76.7% to 92.8% of the propylene conversion, so that the acrylonitrile yield can rise up to 71.2%.
A further reactivation is achieved, by the process of DE-OS (German Published Specification) 3,123,521, in such a way that the recalcined fines of a spent ammoxidation catalyst are added as reactivator in a ratio of 1:3 to a deactivated catalyst. In this way, yield improvements of about 5% can be achieved.
Quite generally, the acrylonitrile yield can of course also be held at a high level or raised from decreased values to a higher level by quantitative or at least partial replacement of deactivated catalyst by fresh catalyst. The disadvantage of this last-mentioned method is represented by the relatively large quantities which are required to produce or maintain this effect. Thus, a quantity of 50% by weight of the catalyst originally charged into the reactor is necessary per year even for only partial replacement.